Known production A/F control systems use a pre-catalyst switching exhaust-gas-oxygen (EGO) sensor to maintain an air-to-fuel (A/F) ratio at a desired average value. As is known in the art, the A/F ratio is ideally at stoichiometry, or in the range of 14.3-14.7 pounds of air to 1 pound of gasoline, depending on the gasoline blend. The stoichiometric ratio is the point at which the gasoline burns completely with no excess carbon monoxide or oxygen. The ratio between the actual A/F ratio sensed by the EGO sensor and the A/F ratio at stoichiometry is called .lambda.. Ideally, .lambda.=1.0 during engine cruising conditions. A .lambda. value of less than 1.0 indicates a rich condition (i.e. there is not enough oxygen to react with the carbon and hydrogen in the fuel), and a .lambda. value of greater than 1.0 indicates a lean condition (i.e. there is too much oxygen and not enough fuel). The ideal .lambda. value can change depending on the specific engine operating condition; for example, the engine may run slightly rich during acceleration and slightly lean during deceleration. If the engine is running too rich or too lean for a particular engine operating condition, however, the catalyst efficiency will decrease, producing undesirable emissions such as HC and CO (during a rich condition) or NOx (during a lean condition).
To maintain the A/F ratio at the optimum value, known pre-catalyst switching EGO sensors use a jumpback and ramp process to maintain the A/F ratio at a desired average value. For example, if the A/F ratio is at a lean condition, the A/F ratio jumps in the rich direction and gradually moves further in the rich direction until the A/F ratio crosses stoichiometry and the pre-catalyst switching EGO sensor senses a rich condition. Once the A/F ratio crosses stoichiometry, the A/F ratio jumps back in the lean direction and gradually moves further in the lean direction until it cross stoichiometry again. This process is used to keep the A/F ratio in the switching range of the switching EGO sensor; known switching EGO sensors only sense whether the engine is running rich or lean, but not the degree to which the A/F ratio deviates from stoichiometry.
A/F ratio control can be improved by including a post-catalyst switching EGO sensor and a bias table in addition to the pre-catalyst switching EGO sensor to form a post-catalyst feedback loop. The post-catalyst switching EGO sensor and the bias table correct small errors in the A/F ratio rapidly by using the bias table to bring the A/F ratio within an estimated range and then using a proportional controller to fine-tune the A/F ratio to its optimum value after the post-catalyst EGO switching sensor has switched states. More specifically, the bias table stores biasing values that correspond to various engine operating conditions (e.g., acceleration, deceleration, idling, etc.) over the speed/load operating range of the engine. The A/F bias values in the bias table are derived empirically. During engine operation, the average A/F ratio is pre-biased as a function of engine operating conditions according to the bias table to maintain the A/F ratio within the optimum range.
Known engine controllers tend to experience two problems, however. First, the proportional feedback term in the engine controller tends to cause undesirable low frequency limit-cycle oscillations in the post-catalyst feedback loop. This oscillation problem often occurs in systems using continuous high proportional gain, which tend to overshoot the desired A/F ratio and reach the desired ratio only after several oscillations occur. Second, because the bias table is derived empirically, the values in bias the table will be optimal only for a particular vehicle. Thus, different vehicles may require different biasing tables. Further, as the vehicle ages, the optimum A/F ratios for that vehicle may change, making the empirically derived bias table values less accurate over time. As a result, a given biasing table may work adequately for multiple vehicles and/or at multiple stages of a vehicle's life, but would not be optimized for any one vehicle. The resulting A/F ratio obtained from such systems would therefore tend to lower the catalyst efficiency and produce higher emissions to some degree.
There is a need for an engine control system that can quickly and accurately reach an optimum A/F ratio, regardless of the engine's type or age, while minimizing undesirable oscillations in the post-catalyst feedback loop.